The present invention relates to a system and a method respectively for dispersion compensation in fibre optic high speed systems. Fibre optic high speed systems are being used in various communication applications, for instance in telecommunication over long transmission distances. A telecommunication network can be divided into separate levels such as for instance subscriber networks, regional networks and inter-regional networks or national networks. These national networks can for instance exist between different cities where there is a demand for very high transmission speeds, for instance 2,5 Gbit/s. However, a limitation in transmission speed between transmitter and receiver occurs due to dispersion. Dispersion is particularly a problem from 2,5 Gbit/s and upwards and at 10 Gbit/s it constitutes a fundamental limitation due to the fact that even the information band width gives rise to a limitation. In conclusion, the dispersion causes problems at high speeds and long distances. The importance of the dispersion thus increases at bit rates of over 2,4 Gbit/s in ordinary single-mode fibres for wavelengths of around 1,55 .mu.m. At repeater-distances of approximately 60 kilometers the effect becomes noticeable at approximately 10 Gbit/s. The demands on analog and digital systems respectively will be partly different due to, amongst other things, the fact that a good linearity is essential in analog systems while it is of minor importance in digital systems.
A number of different systems for dispersion compensation of fibre optic high speed systems have been proposed. A frequently used system is based upon so called prechirp-generation which normally means that the laser is frequency/wavelength modulated during each pulse. Similar systems are described in for instance T. L. Koch, R. C. Alferness, "Dispersion Compensation by Active Predistorted Signal Synthesis". J. of Lightwave Technology, Vol. LT-3, No. 4, (1985), pp 800-805. At 1,05 .mu.m and for an ordinary single-mode fibre it is required that the signal should blue-shift. By direct modulation, a laser normally red-shifts during the pulse. In general, the laser is FM modulated for obtaining the chirp after which the AM modulation is applied by means of an external modulator. In for instance N. Henmi, T. Saito, M. Yagamushi, S. Fujita, "10-Gb/s 100 km normal fiber transmission experiment employing a modified prechirp technique", Proc: OFC'91, (1991), paper Tu02, it is described how selected DFB lasers are utilized. With a so called blue-shift modulation in the transmitter, for instance described in F. Koyoma, K. Iga, "Frequency Chirping in External Modulators, J. of Lightwave Technology", Vol. LT-6, No. 1, (1988), pp 87-93, the FM/AM modulated signal is obtained in an external modulator, the laser functioning without any influence. The required modulation is normally obtained through a special design of the external modulator. Neither a so called prechirp generation nor a blue-shift modulation in the transmitter are in a real sense dispersion-modulating, but instead use the dispersion to bring about a pulse compression. Both of these kinds of systems are mainly used in digital systems where the effect is concentrated towards the middle of the bit gap at the expense of the linearity which, as has been mentioned above, does not have such a great importance in digital systems. The energy of the signal is thus concentrated towards the centre of the bit gap. However, the side-bands in the modulated signal are distorted. Furthermore, a system or a method of this kind implies that the pulse amplitude never becomes so high that the nonlinear range of the medium is entered.
The methods are usually suitable for digital systems providing that the distances generally are not greater than approximately 75 kilometers and the bit rate does not exceed approximately 10 Gbit/s. In conclusion it can be said that the frequency spectrum is distorted in such a way that the pulses in the fibre converge. The system, or the method, is not suitable in analog systems where a greater transmission distance is obtained at the expense of the linearity which is of importance in analog systems.
In accordance with another known system, the transmission is made dispersion-free by introducing an additional length of fibre in which the dispersion has a reversed sign. This is described in for instance H. Izadpanah et al, "Multiwavelength Dispersion Compensation for 1660 nm Transmission at 2,5 Gb/s Over 1310 nm Optimized Single-Mode Fiber", Proc: EPOC'92, (1992), paper TuA5.1. Through this, a real dispersion compensation is obtained in contrast to the above-mentioned method. The system is based on phase compensation of the frequency spectrum of the received signal, and the phase compensation counteracts the phase difference obtained by the different partial frequencies in the laid-out fibre. The dispersion compensation is carried out in the optic domain and is normally achieved by combining different fibre lengths with different signs of the dispersion, after which the transmission medium will become dispersion-free and the optic signal can be detected in a normal way.
Systems where an already laid-out dispersive fibre is being used involve a dispersion compensating fibre having to be arranged before the receiver. The compensating fibre length may be approximately one third of the transmission distance. This results in a number of disadvantages since the extra fibre or fibre length is costly, requires a special design and since it adds attenuation. In the above-mentioned document the losses are limited by adding a fibre amplifier between the transmission fibre and the dispersion compensation fibre, which further complicates the system and raises its cost.
In accordance with another known system, for instance described by J. J. O'Reilly, M. S. Chauldry; "Microstrip Compensation of fibre Chromatic Dispersion in Optically Amplified Coherent Systems in IEE Colloquium on Microwave Optoelectronics, No. 139, (1990), pp 13/1-13/6, the signal is likewise dispersion-compensated at the receiving side. In this way also, a true dispersion compensation is achieved and the system is based upon phase compensation of the frequency spectra of the received signal which counteracts the phase difference obtained by the different partial frequencies in the laid-out fibre. This system requires a coherent technique in the receiver which is also relatively complex and expensive. In the described document the phase distortion is taken care of at an intermediate frequency. A mixer is arranged which consists of an optic direction coupler which is fed by a signal and local oscillator, a detector diode and a bandpass filter and which only lets through the difference frequency. The phase correcting element consists of a microstrip conductor which has a normal dispersion; the microstrip conductor can for instance be 10-20 cm long and compensate for the dispersion in a fibre of a couple of hundred kilometers. When the phase distortion is compensated, the electrical signal is detected by means of traditional methods.